The present invention relates generally to medical devices, and more particularly to implantable artificial cardiac pacemakers adapted to provide patient-variable stimulation rates appropriate to a condition of exercise by the patient.
The resting heart rate of sinus rhythm, that is, the rate determined by the spontaneously rhythmic electrophysiologic property of the heart's natural pacemaker, the sinus node, is typically in the range from about 65 to about 85 beats per minute (bpm) for adults. Disruption of the natural cardiac pacing and propagation system may occur with advanced age and/or cardiac disease, and is often treated by implanting an artificial cardiac pacemaker in the patient to restore and maintain the resting heart rate to the proper range.
In its simplest form, an implantable pacemaker for treatment of bradycardia (abnormally low resting rate, typically below 60 beats per minute (bpm)) includes an electrical pulse generator powered by a self-contained battery pack, and a catheter lead including at the distal end a stimulating cathodic electrode electrically coupled to the pulse generator. The lead is implanted intravenously to position the cathodic electrode in stimulating relation to excitable myocardial tissue in the selected chamber on the right side of the patient's heart. The pulse generator unit is surgically implanted in a subcutaneous pouch in the patient's chest, and has an integral electrical connector to receive a mating connector at the proximal end of the lead. In operation of the pacemaker, the electrical pulses are delivered (typically, on demand) via the lead/electrode system, including an anodic electrode such as a ring behind the tip for bipolar stimulation or a portion of the pulse generator case for unipolar stimulation, and the body tissue and fluid, to stimulate the excitable myocardial tissue.
Pacemakers may operate in different response modes, such as asynchronous (fixed rate), inhibited (stimulus generated in absence of specified cardiac activity), or triggered (stimulus delivered in presence of specified cardiac activity). Further, present-day pacers range from the simple fixed rate device that offers pacing with no sensing (of cardiac activity) function, to fully automatic dual chamber pacing and sensing functions (so-called DDD pacemakers) which may provide a degree of physiologic pacing by at least a slight adjustment of heart rate according to varying metabolic conditions in a manner akin to the natural pacing of the heart. Thus, some DDD pacemaker patients experience an increased pacing rate with physical exertion, with concomitantly higher cardiac output, and thereby, an ability to handle low levels of exercise. Unfortunately, a significant percentage of the pacemaker patient population, who suffer from atrial flutter, atrial fibrillation or sick-sinus syndrome, for example, cannot obtain the benefit of exercise-responsive pacing with conventional atrial-triggered pacemakers. Moreover, the DDD-type pacemakers are complex and costly to manufacture, which is reflected in a higher price to the patient.
It is a principal object of the present invention to provide a relatively simple and inexpensive pacemaker which provides pacing at a desired resting rate, and which is subject to limited control by the patient to provide a desired exercise rate for a preset period of time following which the pacemaker returns to the resting rate.
Various types of rate responsive pacemakers have been proposed which would sense a physiological parameter that varies as a consequence of physical stress, such as respiration, blood oxygen saturation or blood temperature, or merely detect physical movement, and correspondingly adjust the pacing rate. Many of these rate responsive pacemakers may also be relatively complex, and therefore expensive to the patient.
The present invention is directed toward a low cost pacemaker which can be adjusted at will by the patient, subject to the limited amount of control programmed into the device by the physician for that patient. According to the invention, patient control is manifested by bringing an external magnet into proximity with an implanted reed switch associated with the pacemaker. Of course, limited magnet control has been afforded to the patient in the past for some purposes, such as to enable transtelephonic monitoring of the pacemaker functions. Also, techniques are presently available which permit external adjustment of the stimulation rate of the pacemaker after implantation, as by means of a programming unit available to the physician. For obvious reasons, it is undesirable to give the patient the same latitude to control his pacemaker.
In U.S. Pat. No. 3,623,486, Berkovits disclosed a pacemaker adapted to operate at either of two stimulation rates, and switchable from one to the other by the physician using an external magnet. In this manner, the physician would be able to control the pacer mode and rate according to the needs of the particular patient. The purpose, in part, was to provide a pacemaker which had some adaptability to the patient's requirements. However, once set by the physician, the selected resting rate was maintained for that patient by the implanted pacer.
Another technique for external adjustment of pacing rate by the physician is found in the disclosures of U.S. Pat. No. 3,198.195 to Chardack, and U.S. Pat. No. 3,738,369 to Adams et al. In each, rate control is exercised by inserting a needle through a pacemaker aperture beneath the patient's skin to adjust a mechanism. In the Adams et al. disclosure, the needle is used to change the position of a magnet within the paper to actuate a rate-controlling reed switch.
In U.S. Pat. No. 3,766,928, Goldberg et al. describe an arrangement for continuous adjustment of rate by a physician using an external magnet that cooperates with a magnet attached to the shaft of a rate potentiometer in the implanted pacemaker, to provide the initial setting of pacing rate desirable for the particular patient.
More recent proposals offer the patient limited control over the pacing rate. In U.S. Pat. No. 4,365,633, Loughman et al. disclose a pacemaker programmer which is conditioned by the physician to give the patient the capability to select any of three distinct rates: for sleep, for an awake resting state, and for exercise. The programmer generates a pulsating electromagnetic field, and allows the patient to select any of those three modes with an abrupt change in rate when the coil pod of the programmer is positioned over the implanted pacemaker. It is, of course, necessary to have the programmer at hand in order to change the stimulation rate, and the use of the device in public can be a source of extreme embarassment to the patient.
In U.S. Pat. No. 4,545,380, Schroeppel describes a technique for manual adjustment of rate control contrasted with the activity sensing, automatic rate control disclosed by Dahl in U.S. Pat. No. 4,140,132. According to the Schroeppel patent, a piezoelectric sensor and associated circuitry are combined with the implanted pulse generator of the pacemaker to allow the patient to change from a resting rate to a higher rate by sharp taps on his chest near the site of the piezoelectric sensor. Such an arrangement requires that the sensor be sufficiently sensitive to respond to the patient's sharp taps, and yet be insensitive to the everyday occurrences the patient encounters while undergoing normal activities and which could otherwise result in false triggerings. These include presence in the vicinity of loud noise such as is generated by street traffic, being jostled in a crowd, experiencing bumps and vibrations while riding in a vehicle, and the like. Further, even when controlled in the manner described, this type of switching results in an abrupt, non-physiological change of rate.
Accordingly, it is another object of the present invention to provide a pacemaker which is capable of being controlled externally by the patient to assume exercise and non-exercise rate modes, in a manner that allows discreet and yet reliable control.
Yet another object of the invention is to provide a cardiac pacemaker whose stimulation rate is controllable by and according to a schedule selected by the patient.